(176.8) (126.8) (76.8) (26.8) (-23.2) (-73.2)

Depth in Meters

(Altitude)

Bathymetry of Lake Michigan

T.L. Holcombe1, D.F. Reid2, W.T. Virden3, T.C. Niemeyer3,4, R. De la Sierra5, and D.L. Divins3

Contour Interval: 5 meters Transverse Mercator Projection Scale 1:250,000

ABSTRACT

Bathymetry of Lake Michigan has been compiled utilizing the entire historic

sounding data base. Bathymetric contours were scanned and vectorized to

geographic coordinates from scale 1:250000 compilation sheets. This

bathymetry resolves physiography of the lake floor to the extent that known

features are revealed more accurately, and features never before seen are

revealed for the first time. The Mackinac Channel, a subaerial river channel

which drained Lake Chippewa, has a sill depth of about 30m and extends

from north of Garden Island eastward through the Straits of Mackinac. A

drowned fan lying at depths of 50 to 60m dominates the lake floor east of

Washington Island. A large drowned river channel leads upstream from this

fan, across the floor of Green Bay and into Little Bay De Noc. This fan and

channel are evidence of overflow, possibly catastrophic, of Lake Superior into

Lake Michigan via the Au Train-Whitefish Channel during the Lake Chippewa

low stand. Morphology of the Mid-Lake Plateau, a cuesta defended by

eastward-dipping, presumed Devonian limestones, is more accurately

revealed. The Two Rivers end moraine, marking the outer limits of a

significant readvance of retreating Wisconsin ice, extends across the lake

between Manitowoc and Ludington. Apparently shallow lakes connected by

well-defined channels occupied the floor of Green Bay during Holocene

times of lower lake level.

PREPARATION OF LAKE MICHIGAN BATHYMETRY

Bathymetry has been compiled using the entire array of good-quality

historical hydrographic soundings collected in support of nautical charting

over a 120-year period by the NOAA National Ocean Service and its

predecessor agency for Great Lakes surveying, the Army Corps of

Engineers. An estimated 540,000 bathymetric soundings were employed, of

which approximately 60 per cent were already in digital form, 25 per cent

were digitized in conjunction with this effort, and the remaining 15 per cent

were available only on paper survey sheets.

Density of tracklines is generally about 2000m for the open lake and ranges

from 200m to 600m for nearshore areas (see figure below).

Soundings collected since 1903 were already reduced to the Lake Michigan

mean low water datum; these were used for bathymetric contouring without

further calibration or adjustments. Soundings collected prior to 1903 were

reduced to the mean low water datum.

In preparation for bathymetric contouring, digital soundings were converted

to metric units and plotted in color; separate colors were assigned to the

various depth ranges. From the paper sheets, contours in metric units were

generated directly on overlays; these contours were then reduced to the

compilation scale of 1:250,000 and patched in.

Compilation sheets were scanned and vectorized; and the resulting digital

vector bathymetric contour data were used to generate the imagery shown on

the large color plate. Images were constructed using the publicly-available

software "Generic Mapping Tools" (GMT).

MACKINAC CHANNEL

The now-submerged Mackinac Channel was first described by University of

Michigan Professor George M. Stanley (1938) using U. S. Army Corps of

Engineers Lake Survey sounding sheets and some early bathymetric

contours. Stanley was among the first to recognize that significant postWisconsin lowering of water levels in both Lake Michigan and Lake Huron

occurred, with drainage to the sea occurring through an isostaticallydepressed valley in the North Bay area; and that the lowstand in Lake

Michigan (Lake Chippewa) was at a slightly higher water level than that in

Lake Huron (Lake Stanley), with drainage of Lake Michigan occurring

through the Mackinac Channel. The name given to the lowstand in Lake

Huron honors Professor Stanley.

Hough (1955) referred to this channel in the Straits of Mackinac as the

Mackinac River, but in keeping with accepted principles of applying

terminology to water-covered topographic features, we refer to it as the

Mackinac Channel.

Sediment cores from Lake Michigan provided compelling evidence for an

early post-Wisconsin low lake level (Hough, 1955). Further confirmation was

obtained that the Mackinac Channel was a subaerial river channel when

spruce stumps, rooted and in the growth position, were discovered at 35-40m

depth in the Straits of Mackinac. Radiocarbon dates from these stumps

range from 9780 +/- 330 ybp to 8150 +/- 300 ybp; stumps at a depth of 9m

dated 6788 +/- 250 ybp and 6500 +/- 70 ybp have also been recovered from

nearby localities (Somers, 1969; Crane and Griffin, 1972; Lowdon and

Blake, 1978).

This new bathymetry provides an integrated view of the Mackinac Channel

and the adjoining lakefloor topography. Least channel depths of about 35m

occur NW of Waugoshance Point, and at a point north of Garden Island,

where the main channel crosses what may be fall lines formed by resistant

bedrock. A line of ridges, probably bedrock ridges, extends through this

area striking mostly E-W and flanking the main channel. To the east, this

presumed resistant bedrock lineation is on strike with remnants of the

resistant Mackinac Breccia of lower Devonian age, which crop out on Saint

Helena Island and on the peninsula just north of the Straits of Mackinac.

Less resistant shales of the Late Silurian Salina Group underlie the Mackinac

Breccia.

TWO RIVERS RIDGE AND DOOR - LEELANAU RIDGE

An arcuate ridge so named because it is partly underlain by glacial till of the

same name, the Two Rivers Ridge is presumed to be site of an end moraine

marking the outer limits of the last readvance of glacier ice (Two Rivers)

extending this far south in the lake. Till deposits associated with the Two

Rivers readvance underlie the ridge and also crop out on the Wisconsin

shore in the vicinity of the town of Two Rivers, Wisconsin (Lineback and

Gross, 1974).

A foundation of bedrock apparently underlies this ridge, probably composed

of resistant middle Devonian carbonates (Thwaites, 1949; Wold, 1980;

Lineback and Gross, 1974), which have been stripped away by glacial erosion

from the deeper basin to the north. The bedrock core of this ridge with its

north-facing relief probably stalled the readvance of the Two Rivers ice lobe

and ultimately determined the position of the end moraine.

Farther north a smaller ridge extends across the deepest basin of the lake

between Door County, Wisconsin and Leelanau County, Michigan. This

ridge is also arcuate, convex to the south, favoring the suggestion that it,

too, marks the position of an end moraine associated with a minor, even

younger, readvance of the retreating Lake Michigan ice lobe.

CHIPPEWA BASIN AND SOUTH CHIPPEWA BASIN

The largest and deepest basin of Lake Michigan, the Chippewa Basin, extends

northward from the Two Rivers Ridge almost to the outflow point of the

now-submerged Mackinac Channel. It is so named because it is the main site

of the former Lake Chippewa. Depths in excess of 275m, deepest of Lake

Michigan, are reached near the southern end of this basin, where a large

segment of the floor of Lake Michigan extends below sea level.

Bedrock geology of the Chippewa Basin probably consists of a dip slope of

resistant Silurian dolomites forming the western boundary, with the deeper

eastern two-thirds of the basin having been eroded in less resistant upper

Silurian redbeds (see Emery, 1950; Wold, 1980). Evaporites occur within

the upper Silurian section (see Landes et al., 1945), and dissolution of

these evaporites may have contributed to the collapse and stripping away

of the overlying Devonian strata. North-south trending ridges on the floor

of the basin may coincide with erosional remnants of moderately resistant

strata within the upper Silurian section. Escarpments forming the eastern

boundary of the Chippewa Basin probably are underlain by the eroded edges

of the resistant Devonian carbonates.

Whereas the main Chippewa Basin may have been eroded in less resistant upper

Silurian strata, the smaller South Chippewa Basin was probably eroded mostly

in upper Devonian shales, with a dip slope on the west formed partly on

more resistant middle Devonian limestones (Thwaites, 1949; Foster, et al.,

1991). Depths in this smaller basin do not extend below sea level (maximum

depth in excess of 165m), but this basin was deep enough to contain lake

water even during the lowest lake levels of the Chippewa lowstand.

WHITEFISH CHANNEL AND FAN

Spot depths on navigation charts have, since the 1920’s, shown indistinctly

the existence of the large submerged channel beginning in Little Bay de Noc

and extending across the floor of Green Bay and around Washington Island.

To this feature we give the name Whitefish Channel because of its association

with the Whitefish River and the Au Train - Whitefish Valley on land.

Geologists have been speculating for over a century about the history of the

large valley extending northward from Little Bay de Noc across the

peninsula of northern Michigan to Au Train Bay on Lake Superior. These

earlier interpretations presented difficulties because only higher, not lower,

late glacial and postglacial lake levels were assumed. Once the likelihood of

a postglacial lowstand in Lake Michigan was established, University of

Illinois Professor Hough (1955) recognized that the Au Train - Whitefish

Valley, together with the submerged channel extending across Green Bay,

was probably the site of the main outlet of Lake Superior into Lake Michigan

at a time when Lake Michigan level was low.

About 1968, University of Northern Michigan Professor John Hughes

obtained the Lake Survey sounding sheets from the immediate area of the

Whitefish Channel and described the bottom morphology. He discovered the

existence of the large fan, here named the Whitefish Fan, which lies at the

downslope end of the Whitefish Channel and has a top depth in the range of

50 to 55m. Hughes (1989) attributed formation of the Whitefish Channel and

Fan to drainage, possibly catastrophic at times, of Lake Superior into Lake

Michigan when western Lake Superior was open water but eastern Lake

Superior was filled with ice and the St. Marys River outlet was blocked.

Hughes recognized that the level of the top of the fan constitutes a

record of the level of Lake Chippewa at this location.

This new bathymetry gives an integrated view of the channel and fan and the

topography of the surrounding lake floor which is more detailed than that

published by Hughes (1989). It demonstrates that there are no other

submerged fans or channels in northern Lake Michigan of anywhere near

comparable size. The twenty-meter difference between Lake Chippewa level

here and Lake Chippewa level at its outlet is accounted for by the difference

in subsequent isostatic rebound which has occurred between the two

localities.

RIDGES AND VALLEYS IN THE ISLANDS AREA

In the islands area of northern Lake Michigan, a series of distinct N-S

trending ridges and valleys characterize the lake floor. This topography has

the highest local relief of any in Lake Michigan and its relief exceeds 180

meters in places. Overall the topography looks something like that of the

finger lakes region of New York, and the effect of glacial erosion in sculpting

the topography is well known. Grand Traverse Bay and the adjacent land

areas in Michigan have the same ridge and valley topography and the onland features merely represent that portion of the topographic province

which is not submerged. The islands are the emergent portions of ridge-tops.

Many of the ridges are relatively flat top features bounded by steep

escarpments and relatively deep, flat-floored valleys.

Sediment cover of glacial drift and lacustrine sediments is thin over much of

this area, and bedrock outcrops are known to occur or may be expected on

the ridge-tops, the escarpments, and some shallower areas of the lake floor.

This area is underlain by upper Silurian and middle and upper Devonian

strata which dip gently southeastward toward the center of the Michigan Basin.

Hard limestone horizons occur within the Traverse Group, the Dundee

Limestone, and the Bois Blanc Formation. Several islands are underlain

by resistant bedrock of one or more of these three rock units (Thwaites,

1949; Milstein, 1987), and we would expect that many of the prominent

ridges are also underlain by one or more of the three resistant units.

The valleys apparently are areas where glacial erosion has cut through one

or more of the hard Devonian carbonates and exposed underlying upper

Silurian or Devonian shales or redbeds which are not as resistant to erosion

and which furthermore contain evaporites which may be particularly

susceptible to corrosion. The valleys of Grand Traverse Bay apparently are

underlain at their entrance by a dip slope formed on Traverse Group and

Dundee Limestone. Both valleys deepen southward, excavated into the

overlying upper Devonian shales of the Antrim and Ellsworth Formations

(Emery, 1950; Milstein, 1987).

Steepness of the escarpments separating ridges from valleys is noticeable and

could in part be explained by abrupt differences in resistance to erosion

between overlying hard strata and underlying soft shales and redbeds.

Erosion of the escarpments may have been enhanced by coastal erosion

during the Lake Chippewa lowstand, which would have placed tops of some

of the presently submerged escarpments in the shore zone.

MID-LAKE PLATEAU

Referred to as the Mid-Lake High in previous literature, we propose that this

feature be termed the Mid-Lake Plateau in keeping with the established

principles of use of terminology for water-covered topographic features.

This broad, relatively flat-topped cuesta, lying directly east of Milwaukee,

lies generally at depths less than 90m and extends upward to minimum

depths of 40-60m at three localities. Escarpments which form its

northwestern, western, and southwestern boundaries are thought to expose

the resistant eastward-dipping middle Devonian limestones of the Traverse

Group (Thwaites, 1949; Emery, 1950; Wold, 1980).

Deeper channels on the west and east, and the South Chippewa Basin on the

south, bound the Mid-Lake Plateau and isolate it from other shallow water

areas. This feature was undoubtedly an island during part of the time of the

Lake Chippewa lowstand. Post-glacial lacustrine sediments thin or are

missing over the top of this feature, suggesting that it is today swept by

strong currents which prevent or inhibit sediment deposition.

Tills associated with the Manitowoc readvance terminate against the

northwest escarpment (Lineback and Gross, 1974), suggesting that here, too,

occurrence of resistant bedrock has determined the maximum extent of one

of the last readvancing ice lobes during the post-Wisconsin ice retreat.

REFERENCES CITED

Crane, H. R. and J. B. Griffin, 1972, University of Michigan radiocarbon

dates XV, Radiocarbon, v. 14, no. 1, p. 195-222.

Emery, K. O., 1951, Bathymetric chart of Lake Michigan, University of

Minnesota technical paper, no. 77, 11 p.

Foster, D. S., D. W. Folger, C. F. Polloni, and C. L. Brown, 1992, The

stratigraphic framework and distribution of bottom sediment texture -

Southern Lake Michigan, in Folger, D. W., et al., eds., Southern Lake

Michigan Coastal Erosion Study Workshop, St. Petersburg, FL, U.S.G.S.

Open File Report 92-324, p. 24-27.

Foster, D. S. and D. W. Folger, 1994, The geologic framework of southern

Lake Michigan: in Folger, D. W., S. M. Colman, and P. W. Barnes, eds.,

The Southern Lake Michigan Coastal Erosion Study, Journal of Great

Lakes Research, v. 20, no. 1, p. 44-60.

Hough, J. L., 1955, Lake Chippewa, a low stage of Lake Michigan indicated

by bottom sediments, Bulletin of the Geological Society of America, v. 66,

p. 957-968.

Hughes, J., 1989, When Green Bay was a valley: The Au Train-WhitefishGreen Bay spillway, in Palmquist, J. C., ed., Wisconsin’s Door Peninsula,

Perin Press, Appleton, WI, p. 49-65.

Landes, K.K., G.M. Ehlers, and G.M. Stanley, 1945, Geology of the Mackinac

Straits Region and Sub-Surface Geology of Northern Southern Peninsula,

Biennial Report of the Geological Survey Division, State of Michigan,

publication 44, geological series 37, Lansing, MI, p. 123-153.

Lineback, J. A., D. L. Gross, and R. P. Meyer, 1974, Glacial tills under Lake

Michigan (Studies of Lake Michigan Bottom Sediments, no. 11), Illinois

State Geological Survey, Environmental Geology Notes, no. 69, 48 p. (also

University of Wisconsin Geophysical and Polar Research Center

Contribution 311).

Lowdon, J. A. and W. Blake Jr., 1978, Geological Survey of Canada

radiocarbon dates XVIII, Geological Survey of Canada Paper 78-7, 20 p.

Milstein, R. L., 1987, Bedrock Geology of southern Michigan (Geologic map),

State of Michigan, Department of Natural Resources, Geological Survey

Division, scale 1:500,000.

Somers, L. H., 1969, Preliminary report on geological studies in northern

Lake Michigan using underwater observation techniques, in Proc. 11th

Conf. Great Lakes Res. 1968, International Association of Great Lakes

Research, p. 239-244.

Stanley, G. M., 1938, The submerged valley through Mackinac Straits,

Journal of Geology, v. 46, p. 966-974.

Thwaites, F. T., 1949, Geomorphology of the basin of Lake Michigan, Mich.

Acad. Sci. Papers, v. 33, p. 243-251.

Wold, R. J., 1980, Review of Lake Michigan seismic reflection data, U.S.G.S.

Open File Report 80-902, 36 p.

NATIONAL ENVIRONMENTAL SATELLITE, DATA, AND INFORMATION SERVICE

NATIONAL GEOPHYSICAL DATA CENTER

OFFICE OF OCEANIC AND ATMOSPHERIC RESEARCH

GREAT LAKES ENVIRONMENTAL RESEARCH LABORATORY

U.S. DEPARTMENT OF COMMERCE

NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION

NGDC wishes to thank the following individuals for reviewing this poster:

D. Foster, USGS, C.E. Larsen, USGS, and T. Moore, University of Michigan.

1 National Oceanic and Atmospheric Administration

National Geophysical Data Center

Boulder, Colorado

2 National Oceanic and Atmospheric Administration

Great Lakes Environmental Research Laboratory

Ann Arbor, Michigan

3 Cooperative Institute for Research in Environmental Sciences

University of Colorado

Boulder, Colorado

4 Presently with Environmental Consulting, Olancha, CA

5 Cooperative Institute for Limnology and Ecosystems Research

University of Michigan

Ann Arbor, Michigan

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